1. Field of the Invention
This invention relates to a rust preventive steel plate with organic coating for use in automotive parts that has improved cationic electrodeposition quality, workability, weldability, corrosion resistance and resistance to chromium dissolution.
2. Description of the Prior Art
In response to the growing need for increasing the corrosion resistance of automotive steel plates, various types of corrosion preventive steel plates have been proposed and are being gradually accepted by the industry. The first to be mentioned of these corrosion preventive steel plates are plated ones such as those prepared by hot dipping molten zinc or zinc alloys or by electroplating zinc or zinc alloys. However, these plated steel plates are not completely satisfactory for use in curled or hemmed portions of inner plates of car bodies where particularly high corrosion resistance is required on the surface.
Zinc chromated steel plates provided with zinc-rich coatings are known to have high corrosion resistance. However, if such steels having corrosion preventive coatings are subjected to mechanical working such as press forming, the coating can separate from the substrate to cause deterioration in corrosion resistance.
With a view to solving these problems, it was recently proposed that thin organic films (0.3-3 .mu.m) entirely free from electroconductive pigments be formed on the substrate plate of steel plates to make them amenable to subsequent coating by electrodeposition. Such steel plates are described in Japanese Laid-Open (kokai) Application Nos. 62-289274, 63-22637 and 63-35798. These steel plates with organic coatings are improved in many aspects including corrosion resistance, weldability, press formability, appearance after electrodeposition, and the waterproofing secondary adhesion of intermediate coats on the electrodeposited film. However, these improvements can only be achieved when the organic coating is fully crosslinked with a crosslinking agent.
One of the practices gaining popularity today is to make steel plates of "bake hardenable" materials that have low yield strength prior to press forming but that will increase in yield strength upon baking of subsequently coated films. In order to fully exploit the bake hardenability of such materials, the heating of organic coatings for drying and curing them must be performed at temperatures not higher than 150.degree. C. In special cases where high production rates are of primary importance, it is required that the temperature of 150.degree. C. be reached within one minute and that no retention time be provided. These requirements are very strict and unfavorable for the purpose of completely drying and curing the organic coatings. In fact, the conventional organic coatings are made of resin systems that should be fully crosslinked in order to exhibit their intended functions, so they cannot be crosslinked by a satisfactory degree if they are subjected to the low-temperature, rapid heating described above. During subsequent cationic electrodeposition, such insufficiently crosslinked organic coatings will dissolve or become soft upon swelling on account of the alkali that is generated at the interface between the electrodeposited coating and the organic coating, to thereby deteriorate the appearance, paint adhesion and corrosion resistance of the applied coatings.